A sea of green filled Fifth Avenue on Thursday, marking the life of Ireland’s patron saint with the biggest St. Patrick’s Day parade in America and the 250th in New York City.

The parade stepped off at 11 a.m. on Fifth at 44th Street led by best-selling author Mary Higgins Clark as grand marshal.

Parade officials expected about 200,000 marchers and up to 2 million spectators on a sunny March day with temperatures forecast to hit the mid-60s.

New York Mayor Michael Bloomberg marched with Clark, Police Commissioner Raymond Kelly and other city officials.

Roman Catholic Archbishop Timothy Dolan and his predecessor Cardinal Edward Egan greeted the mayor in front of St. Patrick’s Cathedral at 50th Street.

Earlier this month, Bloomberg got a less than warm welcome at a St. Patricks’ Day parade in Queens. Some parade-goers were angry about the mayor’s joke last month that he usually saw “people that are totally inebriated” at the American Irish Historical Society in Manhattan. Bloomberg apologized shortly after making the comment.

Asked about the matter again before Thursday’s parade, Bloomberg said, “I told a joke some people didn’t find funny. But the reception I got so far puts a smile on my face.”

Dolan waved off a question about whether the mayor’s joke had offended him.

“On a day like this, forget it,” he said.

Spectator Michelle Kelly, visiting from Dublin with her husband, Eamon, had tufts of green artificial hair on her head as she waved an Irish flag.

“It’s brilliant,” she said. “I’ve never seen anything like this, not even in Dublin. There are so many different cultures together here — just for the Irish.”

Lawyer Michael Markunis, the great-grandson of Irish immigrants, took time off from his busy work day to watch the parade.

“I’m here because St. Patrick’s Day was a big deal in my family in California,” he said.

There was at least one arrest at the start of the parade as police moved in to stop a fight at 48th Street. A police spokeswoman had no immediate information on arrests.